• A lot of what we know about heredity can be traced to Gregor Mendel
• Often called the ‘father of genetics’ Mendel was one of the first scientists to apply mathematics to scientific investigations.
• He did most of his work with the pea plant, Pisum Sativum. He chose pea plants for the following reasons:
o They are self pollinating (still produce sexually)
o The grow easily and mature quickly, producing many seeds
o Display obvious, contrasting physical traits. Ex. seed shape, plant height, seed colour...
• These physical, observable traits are called the phenotype
• Mendel first developed pure lines or pure breed plants
• He controlled his experiments by making sure he was crossing pure bred plants that were different for only one contrasting trait
Parent Generation (PG)
F₁ Generation (Filial)
Pure tall & pure dwarf All tall
(monohybrides) F₁ Cross
Monohybrid cross 75% tall
3:1 phenotypic ratio
Law of Segregation
• We know now that the units of inheritance are ______(a sequence of DNA) and that different forms of genes are called alleles
• Mendel suggested that each plants phenotype was determined by a pair of alleles (one on each homologous chromosome)
• Mendel used upper case letters to represent the dominant allele and lower case letters to represent the recessive allele.
o The allele for tallness is T , while the dwarf allele is t
• The genetic make up for a trait is called the genotype.
• When the alleles are the same, the genotype is described as homozygous, and when they are different heterozygous.
Explaining Mendel with Meiosis
Mendel’s law of segregation: members of a pair of alleles for a given trait are segregated when gametes are forms
Generation Genotype Phenotype
F TT x tt Tall x dwarf
F₁ Tt All tall
F₂ Tt x Tt
TT, tt, Tt, Tt 75% tall, 25% short